1. Field of the Invention
The invention relates to protection by electromagnetic contactors in general, and more particularly to the use of contactors for the protection of a bidirectional motor.
2. Description of the Prior Art
Electromagnetic contactors are well known. See for instance U.S. Pat. No. 3,339,161 issued Aug. 29, 1967 to J. P. Conner et al. entitled "Electromagnetic Contactor". As opposed to a plain circuit breaker, such as an AC line switchbreaker, a contactor may effect circuit breaking and reclosing operations many times and in close succession, in accordance with several functions such as starting, coasting, switching, overload or overcurrent detection and timed switching. To perform those various functions, a protection control unit is associated with the contactor and, in its more modern form, is built as a solid state circuit.
With a bidirectional motor it is known to combine two such contactors connected and used for establishing alternating motor operation modes, one in the forward direction, the other in the reverse direction, each contactor sharing a common protection control unit.
The main object of the present invention is to use two contactors, each with a separate protection control unit, to provide protection of a bidirectional motor against overheating.
It is known to associate with one contactor a control function performing the determination of the thermal image of the load, i.e. of a motor. See for instance: U.S. Pat. Nos. 4,525,763; 4,547,826; 4,423,458 and 4,423,459. The thermal image is represented by an accumulation of I.sup.2, where I.sup.2 is the square of the current passing through the motor. Whenever a reference value has been exceeded, the contactor is triggered to trip and the AC supply to the motor is disconnected.
The protection control unit can determine, for a motor operating continuously in the same mode, whether to cause the associated contactor to trip if the thermal image reveals a critical stage. With two contactors operating separately for a single bidirectional motor, the problem exists to be able to treat both protection control units as a single unit when determining at any given time whether the thermal image of the heat accumulated in the common motor requires an overall tripping. The present invention aims at solving this problem.